1. Field of the Invention
The present invention relates to a process for preparing a melt-processed organic-inorganic hybrid material and a method of preparing an improved field-effect transistor including a melt-processed organic-inorganic hybrid material. More particularly, the present invention relates to a process for preparing a melt-processed perovskite material and a method of preparing an improved field-effect transistor including a melt-processed perovskite material.
2. Description of the Prior Art
Organic materials have received considerable recent attention as potential replacements for inorganic counterparts in flat panel display driver circuitry and light-emitting elements, as well as for enabling technology for flexible and low-cost electronic devices. Organic materials have the advantage of simple and low-temperature thin-film processing through inexpensive techniques such as spin coating, ink jet printing, or stamping. In addition, the flexibility of organic chemistry enables the formation of organic molecules with useful luminescent and conducting properties. Since the first consideration of organic electroluminescence (EL) devices over 30 years ago (J. Dresner, RCA Rev. 30, 322 (1969)), organic light-emitting devices (OLEDs) have been widely pursued and near-commercial dot-matrix displays have recently been demonstrated (T. Wakimoto, et. al., J. Soc. Info. Display 5, 235 (1997)). In addition to emitting light, the semiconducting properties of some organic materials enable promising technologies for organic field effect transistors (OFETs). Over the last few years, the carrier mobilities of organic channel layers in OFETs have increased dramatically from <10−4 to ˜1 cm2/V-sec (comparable to amorphous silicon) (S. F. Nelson, et. al., Appl. Phys. Lett. 72, 1854 (1998) and C. D. Dimitrakopoulos, et. al., Science 283, 822 (1999)).
While promising with regard to processing, cost, and weight considerations, organic compounds generally have a number of disadvantages, including poor thermal and mechanical stability. Electrical transport in organic materials has improved substantially over the last 15 years. However, the mobility is fundamentally limited by the weak van der Waals interactions between organic molecules (as opposed to the stronger covalent and ionic forces found in extended inorganic systems). In OLEDs, the stability and mobility limitations lead to reduced device lifetime. For OFETs, the inherent upper bound on electrical mobility translates to a cap on switching speeds and therefore on the types of applications that might employ the low-cost organic devices. If these issues could adequately be addressed, new technologies might be enabled by alternative semiconductors, including light, flexible displays or electronics constructed entirely on plastic.
Organic-inorganic hybrid materials, including particularly materials of the perovskite family, represent an alternative class of materials that may combine desirable physical properties characteristic of both organic and inorganic components within a single molecular-scale composite.
The basic structural motif of the perovskite family is the ABX3 structure, which has a three-dimensional network of corner-sharing BX6 octahedra. The B component in the ABX3 structure is a metal cation that can adopt an octahedral coordination of X anions. The A cation is situated in the 12-fold coordinated holes between the BX6 octahedra and is most commonly inorganic. By replacing the inorganic A cation with an organic cation, an organic-inorganic hybrid perovskite can be formed.
In these ionic compounds, the organic component is an intimate part of the structure, since the structure actually depends on the organic cation for charge neutrality. Therefore, such compounds conform to specific stoichiometries. For example, if X is a monovalent anion such as a halide, and A is a monovalent cation, then B should be a divalent metal. Layered, two-dimensional A2BX4, ABX4 and one-dimensional A3BX5, A2A′BX5 perovskites also exist and are considered derivatives of the three-dimensional parent family.
The layered perovskites can be viewed as derivatives of the three-dimensional parent members, with y-layer-thick cuts, i.e., y=1, 2, 3 or more, from the three-dimensional structure interleaved with organic modulation layers. The layered compounds generally have inorganic layers with either <100> or <110> orientation relative to the original three-dimensional perovskite structure.
One <100>-oriented family of organic-inorganic perovskites has the general layered formula:(R—NH3)2Ay−1MyX3+1 where M is a divalent metal, X is a halogen atom (i.e. Cl, Br, I), A is a small inorganic or organic cation (e.g. Cs+, CH3NH3+), R—NH3+ is a larger aliphatic or aromatic mono-ammonium cation, and y is an integer defining the thickness of the inorganic layers. In this system, the ammonium group is hydrogen-bonded and ionically bonded to the inorganic sheet halogens, with the organic tail extending into the space between the layers and holding the structure together via Van der Waals interactions.
The (R—NH3)2MX4 (y=1) members of this family include the simplest and most numerous examples of organic-inorganic perovskites. Similar y=1 (or higher y) layered perovskite structures can also be stabilized by diammonium cations, yielding compounds with the general formula (NH3—R—NH3)MX4. In these systems, there is no Van der Waals gap between the layers since the ammonium groups of each organic layer hydrogen bond to two adjacent inorganic layers.
D. B. Mitzi, Prog. Inorg. Chem., 48, 1 (1999) reviews the state of the art and describes organic-inorganic perovskites that combine the useful properties of organic and inorganic materials within a single molecular-scale composite. U.S. Pat. No. 5,882,548 to Liang et al. describes solid state preparation of perovskites based on divalent metal halide sheets. U.S. Pat. No. 6,180,956 B1 to Chondroudis et al. and C. R. Kagan et al., Science, 286, 945 (1999) describe integrating the self-assembling nature of organic materials with the high carrier mobilities characteristic of inorganic materials for possible use in Organic-Inorganic Field-Effect Transistors (OIFET's). A semiconductor-metal transition and high carrier mobility in the layered organic-inorganic perovskites based on a tin(II) iodide framework have also been described. These materials may be used as channel materials for field-effect transistors. K. Chondroudis et al., Chem. Mater., 11, 3028 (1999) describe single crystals and thin films of the hybrid perovskites, which can be employed in Organic-Inorganic Light-Emitting Devices (OILED's).
The organic-inorganic hybrid materials, such as perovskites, may be processed to produce organic-inorganic perovskite crystals or thin films by conventional methods including the solution-based or evaporative techniques described by D. B. Mitzi in the previously cited Prog. Inorg. Chem., 48, 1 (1999) and by Liang et al. in U.S. Pat. No. 5,871,579. However, these methods suffer from being high cost processing methods and generally require the use of environmentally hazardous solvents.
Accordingly, it is an object of the present invention to provide low-cost, melt-processed organic-inorganic hybrid materials, which can be used in a variety of applications, including flat panel displays, non-linear optical/photoconductive devices, chemical sensors, emitting and charge transporting layers in organic-inorganic light-emitting diodes, organic-inorganic thin-film transistors and as channel layers in organic-inorganic field-effect transistors.
These and other objects of the present invention will become apparent by the novel perovskite compositions and the methods of preparing the perovskite compositions.